1. Field of the Invention
This invention relates to a method and a device for recovering horizontal synchronization from a received TV signal by means of a phase-locked loop, comprising the steps of generating a recurring oscillatory signal, phase-comparing said oscillatory signal with horizontal synchronization pulses of said received TV signal, low-pass filtering the result of the phase comparison, and using the filtered result of the phase comparison to control the generation of the oscillatory signal, so as to reduce the phase error between the oscillatory signal and the horizontal synchronization pulses of the received TV signal.
2. Description of the Related Art
The publication "Valvo Technische Information, 841221", entitled "Die Horizontal- Vertikal Kombination TDA 2579," issued by Philips Semiconductor Division, describes a prior art horizontal synchronization recovering method. The purpose of the phase-locked loop is to synchronize an oscillatory output signal, which may, for instance, be used to control the scanning of a picture display with the synchronization pulses of a received TV signal. However, this signal is often severely distorted due to low S/N-ratio, echoes, reduced synchronization pulse amplitude, co-channel interference, etc. In the phase-comparing step, the phase angle of the synchronization pulses of the received TV signal is compared with the phase of the oscillatory signal. The phase-comparing step generates a signal which is proportional to the phase difference. In general, this signal consists of a steady state (DC) component which serves to control the generation of the oscillatory signal, and a fluctuating (AC) component which is caused by the unwanted distortions and which has to be suppressed as much as possible by the low-pass filtering.
In the process of suppressing the distortions, the bandwidth of the phase locked loop, which is inter alia determined by the bandwidth of the low-pass filter, is of crucial importance. The smaller this bandwidth is, the better the distortions are suppressed and the more stable the generated oscillatory signal will be. However the smaller the bandwidth is, the more difficult it is for the phase-locked loop to acquire the locked state. The locked state is preceded by a "catching" state which is the state wherein the frequency and/or the phase of the oscillatory signal is so far away from those of the input signal that it is impossible for the phase-locked loop to become locked. During the catching state, the synchronization pulses of the input signal are usually arranged to directly synchronize the oscillatory signal, of course with the oscillatory signal having all the fluctuations caused by the distortions of the input signal and by the inaccuracy of the measurement. One method of direct synchronization is to apply the input signal, or at least the synchronization pulses of the input signal, directly to the oscillator thereby bypassing the phase detector and the loop filter. Another method which is e.g., known from U.S. Pat. No. 3,846,584 is to switch the loop filter off, so that its bandwidth is very large and the synchronization pulses of the input signal reach the oscillator unfiltered. When the input synchronization pulses and the oscillatory signal sufficiently coincide, the phase-locked loop is switched into "locked" operation.
Practical phase-locked loops for the horizontal synchronization of TV-signals have a loop filter bandwidth of several hundreds of Hertz., e.g., 200 Hertz. This is a compromise between reasonably stable operation of the oscillator in the locked mode, on the one hand, and a reasonably easy and quick transition from the catching state into the locked state, on the other hand. If the bandwidth is much smaller, the locked state can only be entered when the frequency and phase of the oscillatory signal are very close to the frequency and phase of the incoming synchronization pulses. This is very hard to achieve in the catching state.